Water purification plant

ABSTRACT

Water cleaning device with at least one reactor device (4) for electrolytic cleaning of polluted water, assembled of one or more anodes (1a) and cathodes (2a), one flotation device (14, 15) and one device for handling sludge (23, 24, 18). The water flow inside and just outside (downstream) of the electrochemical reactor (4) is turbulent. The turbulence is created by one or more mechanical devices which affect the flow pattern. Examples of such mechanical devices may be a corrugated or rough, an exchangeable inlay plate (5) on the cathode surface, an inert, non-conducting rotating or stationary piece (20) in the narrow space between anode and cathode, a pulsating inlet flow of polluted water, recirculation of processed water (from outlet to inlet of the electrochemical reactor), a static mixer just outside (downstream) the electrochemical reactor, that the water enters through several slanted/inclined channels in the cathode, opposite rotational directions of anode and cathode or a combination of several of these methods. The cathode has a shaft or stud (3) penetrating the anode to facilitate rotation of the cathode. The electrochemical reactor(s) are mounted at the side of the flotation device at the same elevation as the water level. The flotation device is arranged as a tank (14) or a helical system (15), and the device for handling the sludge is a rolling band (23) with adjustable flaps (24) or a phase separator (18) located at the outlet of the helix (15).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention concerns a water cleaning system comprising at least onereactor device for electrolytic cleaning of polluted water, oneflotation device where the pollutants are separated from the cleanwater, and one device for handling the sludge. The polluted water entersbetween at least one anode and one cathode across which a voltage isbeing supplied such that the anode is being continuously sacrificed whenan electric current flows through the electrochemical reactor. From thereactor the water flows into the flotation device where the precipitatedpollutants flotate as sludge to subsequently be dealt with in the sludgehandling device.

2. Prior Art

One problem with reactor designs of the kind described above is that theanode surface, which may be of aluminum, magnesium or another suitablemetal or alloy, gets fouled by pollutants and oxidation products duringthe electrolysis. This lowers capacity and reduces efficiency. Attemptshave been made to solve this problem by rotating the anode (EP-31614),by alternating the polarity on the electrodes (DE-4315117/EP-623558), byhaving very high pressure on the inflowing water (U.S. Pat No.4,236,990), by a distance piece between anode and cathode for mechanicalscraping the anode clean (SE-470423), or by using a high pressurizedliquid for flushing of the anode surface (SE-470554).

Another problem is to obtain sufficiently high separation efficiencywithout high consumption of anode material and energy. This is attemptedsolved by having a short distance between the electrodes(NO-143147/FI-55166 and SE-470554) or by mounting the anode within acylindrical shaped cathode and create powerful agitation or rotation ofthe water to get a centrifugal effect in addition to the electricalfield effect (U.S. Pat No. 5352343).

Even though the problem of anode fouling is partly solved by

SE-470554 and the distance between the electrodes is kept relativelyshort, which lead to a relatively low power consumption, it is notpossible at present to combine high separation efficiency and a lowconsumption of anode material per cubic meter cleaned water. This leadsto higher operating costs, than necessary.

SUMMARY OF THE INVENTION

The basis for this invention is introduction of changes which willimprove the electroflocculation process. The objects of the inventionare to increase both the separation efficiency and the hydrauliccapacity and at the same time reduce consumption of anode material andspecific energy consumption per cubic meter. The device can be used toclean produced water and drainage water from oil- and gas platforms,rolling emulsions, tall oil emulsions, water from scrubbers for cleaningof gas etc., where the components to be removed are mineral oil, talloil, heavy metals and other components which are harmful to theenvironment. The concentration of these components in the cleaned watershall be well below the limits set by the environmental authorities fordischarge of water to a recipient.

These aims can be obtained by a new design of the electrochemicalreactor and the associated device by:

Increasing the turbulence of the water in and just after theelectrochemical reactor (by using a turbulence generator or by arrangingthe electrodes in a different way).

Changing the design of the flotation device and the device for removingsludge.

Changing the location of the electrochemical reactor relative to theflotation device.

The water cleaning system in accordance with the invention ischaracterised by the features mentioned in the requirements shown belowand by having the reactor device assembled from one or more anodes andcathodes where the flow of water inside and downstream of theelectrochemical reactor is turbulent, a flotation device where thepollutants are separated from the clean water, and a device for handlingthe sludge.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail in the text which follows, andthe text which shows some examples of operating principles refer to theenclosed drawings:

Turbulence Generators

FIG. 1 shows a sketch of a corrugated or rough replaceable/exchangeableinlay plate on the cathode surface.

FIG. 2 shows a sketch of a rotatable non-conducting and inert element.

FIG. 3 shows a sketch of a stationary non-conducting and inert element.

FIG. 4 shows the principle of recirculating the processed water throughthe electrochemical reactor.

FIG. 5 shows a sketch of a static mixer downstream of theelectrochemical reactor.

FIG. 6 shows a sketch of the cathode with multiple, slanted/inclinedinlets of water to the electrochemical reactor.

Arrangements of Anodes and Cathodes.

FIG. 7 shows a sideview of an electrochemical reactor where the cathodehas a stud running through the anode to allow rotation of both anode andcathode.

FIG. 8 shows a sideview of the cathode with the water inlet pipes and ahigh pressure flushing equipment provided with rotating couplings.

FIG. 9 shows a sideview of a double anode with a cathode in the middle.

Location of the Electrochemical Reactor Relative to the FlotationDevice.

FIG. 10 shows the electrochemical reactor placed outside the flotationdevice at the same height as the water level.

Flotation Device and off-scraping of Sludge

FIG. 11 shows in perspective a sketch of a flotation tank with aroller-band for scraping off sludge.

FIG. 12 shows a sideview of a helical system provided with a phaseseparator at the outlet from the helical system.

The drawings are not everywhere made to the same scale. To simplify thedrawings, details that are not required in order to understand theprinciple are left out.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The water cleaning system according to the invention includes anelectrolytic reactor device for cleaning polluted water containingmineral oil, tall oil, heavy metals and other polluting components.

FIG. 1 shows a cross section of the anode (1a) and cathode (2b) withcorrugated inlay plate (5) attached to the cathode surface (2b). Thefigure also shows a sketch of the inlay plate (5) seen from above,giving an example of how the structure of the inlay plate (5) may be.The inlay plate creates turbulence in the water (8b) which enters theelectrochemical reactor (4) such that precipitated aluminum from theanode surface (1b) (in the case when the anode material is aluminum)reacts very fast with the pollutants in the water.

FIG. 2 shows a cross section of the anode (1a) and the cathode (2a) witha rotating inert and non-conducting element (6) inside theelectrochemical reactor (4). The rotating element (6) is shaped like aperforated disk such that the pollutants and precipitated aluminum fromthe anode surface (1b) are not prevented from reacting with each other.The element (6) should be thin, preferably 0.3-0.6 mm, in order not totake up too much of the volume inside the electrochemical reactor (4).For design reasons the element (6) should preferably be circular with adiameter approximately equal to that of the anode (1a). The rotatingaction of the element creates turbulence in the water, therebyincreasing the reaction speed in the reactor (4). The element (6) may besupported close to the center axis of the anode (1a) and the cathode(2a), but not necessarily precisely along the center axis.

FIG. 3 shows a cross section of the electrochemical reactor (4) with astationary inert and non-conducting part (20). The part (20) causes thewater to flow a longer distance through the reactor (4), and the waterwill therefore be well mixed with the precipitated aluminum (or othermaterial) from the anode surface (1b). The part(20) is also shown viewedfrom above. The water is here forced to frequently change direction,thus contributing to turbulent flow.

FIG. 4 shows a cross section of the anode (1a) and the cathode (2a)together with a recirculation loop for the processed water (9). Therecirculated flow rate may be of the order 1-10 times the flow rate intothe process (8b). The recirculation increases the flow through thereactor (4), thereby increasing turbulence and consequently the reactionrate. The amount of consumed aluminum pr. unit volume of incoming water(8b) has not changed compared to the process without recirculation,provided the flow rate is constant, but the separation efficiency willbe higher.

FIG. 5 shows a cross section of the electrochemical reactor (4) with astatic mixer (21) mounted outside the node (1a) on top of the cathodesurface (2b), but without being in contact with the anode. The staticmixer (21) increases the turbulence just outside the reactor (4) by theintricate flow path imposed on the water stream.

FIG. 6 shows a cross section of a cathode (2a) where all or parts of thepolluted water flows into the electrochemical reactor (4) throughmultiple slanted/inclined channels (8a) in the cathode. The channels mayhave different angles with the horizontal plane.

FIG. 7 shows a cross section of an electrochemical reactor (4) where thecathode (2a) has an axle or stud (3) going through the anode (1a)thereby making it possible to rotate the cathode (2a) by means of adriving mechanism (motor or similar device). The water (8b) flows intothe reactor (4) through a channel (12) penetrating the anode (2a) andoutside the cathode stud. To prevent electrolysis from taking place inthis channel (12) the surface of the anode (1a) inside the channel (12)may be furnished with an isolating layer (13) which end will stay flushwith the anode surface (1b) even when the anode material (1a) is beingconsumed, or the stud (3) may be made of a non-conducting material ormay have a non-conducting surface.

FIG. 8 shows a cross section of the electrochemical reactor (4) withinlet pipes for water (8a) and high pressure flushing (7) the rotateablecouplings (10). Having rotateable couplings (10) can make the cathode(2a) rotate in the opposite direction of the anode (1a) and therebycontribute to increase the turbulence in the reactor (4). The pipe forthe high pressure flushing (7) and incoming water (8a) are arranged suchthat one pipe runs inside the other pipe a certain distance along thecentral axis of the electrochemical reactor (4). Both inlet pipes (7,8a)have rotatable couplings located where the pipes run along the centralaxis of the reactor (4). The pipes (7, 8a) separate such that one orboth can be unsymmetrically fastened to the rotating anode (2a), andthere is a tight fitting (11) between the pipes (7,8a) where theyseparate and in the rotateable couplings (10).

FIG. 9 shows a cross section of a dual reactor (4) having two anodes(1a) with a cathode (2a) in the middle. the water flows into the reactoras shown in FIG. 7, the only difference being that the water (8b) ispumped into the lowest reactor (4) from underneath rather than fromabove. Processed water (9) from the reactors are mixed before being fedto the flotation device.

FIG. 10 shows in perspective a sketch of the anode (1a) and the cathode(2a) located on the outside of the flotation tank (14) such that theprocessed water (9) from the reactor (4) enters the tank (14) at thesame elevation as the water level (14) or just below. When the waterfrom the reactor (4) flows into the tank (14) just below the water levelin the tank (14), the flotation rate is faster than with traditionalsolutions because gas bubbles and the pollutants have a shorter way totravel.

FIG. 11 shows in perspective a sketch of a flotation tank (14) with avolume equal to 1/5 (=20%) of the average hourly flow rate of water. Theratio between the average hourly flow rate and the cross sectional areaof water in the tank is about 5 m³ /m² h, and the bottom (22) of thetank has the shape of at least one cone. The angle between the walls inthe cone and horizontal is about 60°. The device for handling sludge(23) is shaped like a rolling band with one or more adjustable andflexible scrapes or flaps (24). The rolling band (23) operates in thesame direction as the flow into the tank.

FIG. 12 shows another sideview of a flotation device having the shape ofa helical tube (15). Water (9) from the reactor (4) enters at top of thehelix (15) and is slung out towards the walls because of the rotationthat takes place when the water flows downwards.

The secondary whirls give a speed profile towards the helical walls (15)such that the water meets a large effective area (9). The clean water(16) and the pollutants (17) emerge as two phases at the outlet of thehelix (15) with the water at the bottom. The figure also shows a phaseseparator (18) at the outlet of the helical system which leads the water(16) and the sludge (17) into different paths. Also in this design willthe flotation rate be fast because of short distance to travel for gasbubbles and pollutants.

Increased turbulence in the water just after the electrochemical reactorcauses precipitated aluminum and pollutants to mix more efficiently andreact with each other into sludge. The chemical reaction in the cellconsists of three steps having different reaction speeds.

Precipitation of aluminum from anode to the liquid (10⁻⁴ sec.).

Diffusion of aluminum/Al-complex and pollutants towards each other (0.5sec).

Reaction of aluminum/Al-complex and pollutants to form flocs.

If the flow is laminar, step 2 will determine the reaction speed and itis necessary to sacrifice much aluminum or other anode material to makesure that all the pollutants in the water react. The processed waterwill therefore contain a certain amount of excess aluminum which has notreacted. By increasing the turbulence it is therefore possible to reducethe consumption of anode material and still increase the separationefficiency. The residence time can also be shortened resulting in anincreased hydraulic capacity.

During flotation, however, we strive to obtain as calm flow conditionsas possible so that the flocs will not be damaged, but flow unimpeded tothe surface by means of the hydrogen gas being formed at the cathode. Bymounting the cell as shown in FIG. 10, the hydraulic capacity can alsobe increased because of faster flotation and reduced residence time inthe flotation device.

If the flotation device is in the shape of a tank the values for volume,cross sectional area and sinking speed of the water have to beoptimised. Large cross sectional area and large tank volume results inlong residence time and good flotation, but has high space requirements.With high sinking water velocity the tank volume can be made smaller,but a drag effect will tend to pull sludge down from the surface. Theoptimum values are a tank volume of 1/5 of the average hourly flow ofwater and a water sinking velocity (ratio between volumetric flow rateand cross sectional area) of 5 m³ m² h. If the volumetric flow rate is1m³ /h, the tank volume will be 200 litres and the cross sectional area0.2 m. The cone should have an angle of at least 60° with the horizontalso that heavy sludge-can sink to the bottom and be removed. The sludgescraping device should operate in the direction of the inlet water flowto get as calm conditions in the tank as possible.

By using a helix as a flotation device rather than a tank, the spacerequirements can be reduced. Water is drawn against the wall of thehelical tube because of the rotational flow. Secondary whirls give aspeed profile towards the wall so that the effective surface becomeslarge, and this makes the two phases, water and sludge, to separate fastand the residence time can be reduced. The helix also gives increasedseparation at low speed gradients. At the outlet of the helix the sludgewill lay at the top and the clean water at the bottom, and by using a"knife" adjusted to the correct height, it is possible to separate theto phases two different tanks.

EXAMPLE

Table 1 shows results from experiments with and without a turbulencegenerator. The experiments were done with a prototype from SE-470551with and without a corrugated inlay plate on the cathode. The water usedfor the experiments was so-called ejector water from tall oil productioncontaining monoterpenes, sesquterpenes, diterpenes, fatty acids, resinacid and Na-sulfate. This emulsion is very stable and difficult toseparate. The consumption of anode material pr. m³ water is thereforemuch higher than what is normal for separating other emulsions. Thecontent of pollutants is measured as turbidity (FTU) and chemical oxygenconsume (KOF)(mg/l).

                                      TABLE 1                                     __________________________________________________________________________    Experiments on ejector water from tall oil                                    production with and without a turbulence                                      generator                                                                                                     Turbidity                                                Vol.       Energy       Outlet                                                                             KOF                                              flow                                                                             Voltage                                                                           Current                                                                           consumption                                                                         Dosage                                                                            Inlet                                                                            creduction                                                                         In.                                                                              Red.                               Date No Plate                                                                            m.sup.2 /h                                                                       V   A   k Wh/m.sup.2                                                                        g Al/m.sup.1                                                                      FTU                                                                              %    mg/l                                                                             %                                  __________________________________________________________________________    17/10-95                                                                           1  no 0.50                                                                             25  210 10.5  141 270                                                                              47,4 7273                                                                             16,0                               20/10-95                                                                           JE2                                                                              yes                                                                              0.57                                                                             35  253 15.5  149 296                                                                              74,3 -- --                                 __________________________________________________________________________     (--: values which are not measured)                                      

The table shows that the separation efficiency increases dramatically(from 47.4% to 74.3%) by using the corrugated inlay plate whenconsumption of anode material and other parameters are kept atapproximately the same level. This shows the importance of good mixingin the electrochemical reactor to avoid excessive use of anode materialand therefor higher operating costs to ensure a high degree ofseparation.

The invention can be modified and changed in different ways. Inparticular it must be pointed out that one or more of above mentionedcharacteristics can be combined.

One or both electrodes may be rotatably mounted, and the constructionalmaterials may be quite freely selected if only the constructionalstrength is sufficient high.

What is claimed is:
 1. Water cleaning device comprising at least onereactor device for electrolytic cleaning of polluted water, a flotationdevice for separating pollutants from the polluted water and a devicefor handling sludge, wherein the reactor device has a reaction area andat least one substantially horizontal anode and a cathode parallelthereto located upstream of the flotation device along a flow, having atleast one separate turbulence generator for affecting a flow pattern ofthe water causing the water to flow in a turbulent manner inside thereaction area and in an area immediately thereafter along the flow. 2.Water cleaning device according to claim 1, wherein the turbulencegenerator comprises at least one of a corrugated and rough, exchangeableinlay plate on a surface of the cathode.
 3. Water cleaning deviceaccording to claim 1, wherein the turbulence generator comprises anon-conducting, rotatable piece in the reactor device.
 4. Water cleaningdevice according to claim 1, wherein the turbulence generator comprisesan inert, non-conducting, stationary piece in a narrow space between theanode and cathode.
 5. Water cleaning device according to claim 1,wherein the turbulence generator comprises a static mixer just outsidethe reaction area.
 6. Water cleaning device according to claim 1comprising a means for recirculating water from an outlet to an inlet ofthe reactor device in at least one of a controlled and pulsating manner.7. Water cleaning device according to claim 1, comprising a dual anodewith a cathode in the middle.
 8. Water cleaning device according toclaim 1, comprising a single reactor device, arranged alongside theflotation device at a same elevation as a water level within theflotation device, such that the water has a short distance to travelfrom the reactor device to the flotation device.
 9. Water cleaningdevice according to claim 1, further comprising a helical pipe systemand wherein rotation of polluted water within the helical pipe systemseparates the polluted water into two phases.
 10. Water cleaning deviceaccording to claim 9, wherein the device for handling sludge is equippedwith a phase separator located at an outlet of the helical system. 11.Water cleaning device according to claim 1, comprising at least tworeactor devices, wherein each reactor device is arranged alongside theflotation device at a same elevation as a water level within thefloatation device, such that the water has a short distance to travelfrom the reactor device to the flotation device.
 12. Water cleaningdevice according to claim 1, comprising at least two reactor devicesarranged within the floatation device, wherein at least one of thereactor devices is submerged in water within the floatation device, suchthat the water has a short distance to travel from the reactor device tothe flotation device.